1. Field of the Invention:
The present invention relates generally to the field of frequency synthesizers and, more particularly, to high speed frequency synthesizers of the types commonly used in fast frequency hopping systems.
2. Description of Related Art:
The security and integrity of electronically transmitted information, including communications, is of extreme importance in many situations, both military and civilian. The ever-increasing sophistication of eavesdropping and interference techniques demands correspondingly improved and sophisticated secure communication apparatus and procedures on a continuing basis.
Typically, secure and anti-jamming or anti-interference communication systems employ some type of transmission signal transformation. An important type of transformation frequently used involves the periodic shifting of operating frequencies. For highly secure systems, the frequency shifting or hopping may be at a rate of thousands of hops per second, with the hopping being among many different frequencies in a relatively broad frequency band. To be effective against eavesdropping or interference, the frequency hopping is ordinarily on a pseudo-random basis; that is, the "random" frequency hopping schedule is known beforehand to the parties using the frequency hopping system or is incorporated in some manner into the equipment.
As mentioned above, system security and integrity is improved by increasing the frequency hopping rate and/or the number of frequencies used. As a result, one of the problems associated with high speed frequency hopping systems relates to generating a sufficiently large number of operating frequencies and in enabling the rapid transitions among these frequencies. One manner of providing the different frequencies is by using an individual crystal controlled oscillator for each frequency. In this case, it is only necessary to switch the different frequency oscillators "on-line" and "off-line" as the particular frequencies are used or not used. However, the use of a separate oscillator for each available (programmed) frequency is, for systems using a large number of hopping frequencies, very costly. Moreover, the cost, the weight and physical size of the resulting equipment is excessive for the typical mobile or transportable frequency hopping system.
As an alternative to using discrete frequency generators, most frequency hopping systems use frequency synthesizers; for example, voltage controlled oscillators (VCOs), which can generate (i.e., synthesize) frequencies over a broad band by means of a variable input control voltage.
Although use of frequency synthesizers is generally an improvement over use of discrete oscillators, at least for a many-frequency system, serious problems with fast frequency hopping have remained. A principal problem associated with the use of frequency synthesizers is that, for any particular control condition, such as a specific control voltage, the exact same frequency is not synthesized every time. Thus, due to such factors as aging of electronic components used in the synthesizer or equipment temperature changes, a specific control signal or command may result in the synthesis of one frequency at one time and slightly different frequencies at other times. Moreover, some or all of the synthesized frequencies might be outside of the required tolerance of the intended frequency.
Feedback is, therefore, generally provided in at least those frequency synthesizers which are required to rapidly hop among frequencies. Typically, a closed loop feedback is provided which enables comparing the synthesized frequency with the commanded frequency and which, responsive to the difference between the synthesized and commanded frequencies, adjusts the control signal within the synthesizer in a manner causing the synthesized frequency to converge towards the commanded frequency. Usually a number of frequency periods are required before the closed loop synthesized frequency converges into the tolerance range of the commanded frequency. Also, the closed loop frequency adjustments are intentionally made "on-line," i.e., during the same interval that the synthesizer frequency is being used for some purpose.
In known closed loop feedback frequency synthesizers, each time that a previously selected programmed frequency is re-selected, the corrective feedback adjustment of the control signal is repeated. Stated differently, when a programmed frequency is re-selected for synthesis closed loop frequency synthesizes do not have the capability for "learning" from past experience what corrective adjustments to the control signal should be applied.
Because the closed loop feedback adjustment is repeated each time a different frequency is selected for synthesis, the hopping rate among different frequencies is limited, and "dead time" must be allowed for a single synthesizer to settle. High speed frequency hopping systems are, as a result, often required to use several closed loop frequency synthesizers in a rotationally operated arrangement, often referred to as a carousel arrangement. In a typical carousel arrangement, such as one using four closed loop frequency synthesizers, each synthesizer is "on-line" only 25 percent of the time and is "off-line" while the three remaining synthesizers are sequentially "on-line". At the beginning of its "off-line" portion of the cycle, each carousel-arranged synthesizer is provided the control signal for its subsequent "on-line" portion of the cycle so that the above-described feedback adjustment of the oscillator control signal is substantially or totally completed for a synthesizer before it is switched "on-line."
Although such carousel arrangements of frequency synthesizers in a frequency hopping system typically enable quite rapid frequency hopping, the use of several frequency synthesizers, together with associated selection and control circuits, significantly increase system cost as well as size and weight over that for a system achieving the same performance using only one frequency synthesizer. Improvements in high speed frequency synthesizers used in frequency hopping systems are, therefore, needed to improve performance and reduce system cost, size and weight.
Accordingly, an object of the present invention is to provide a novel, high speed frequency synthesizer having the capability for generating and applying frequency control signal corrections based on synthesized frequency errors determined during previous uses of that same "channel."
Another object of the present invention is to provide a novel, high speed frequency synthesizer which has a memory for storing a set of frequency control commands, each corresponding to a different frequency to be synthesized, and an open feedback loop which enables each control command to be correctively modified for future use in accordance with the measured or estimated error between a frequency actually synthesized and the corresponding commanded frequency.
Other objects, features and advantages of the present invention will become apparent from the following detailed description.